A wire cut electric discharge of this type is as shown in FIG. 1. In FIG. 1, reference numeral 1 designates a workpiece to be machined; 2, a movable table on which a workpiece is mounted; 3, a wire cut electric discharge machine body which operates to move the movable table 2 to machine the workpiece 1 with a wire electrode 4 and a machining solution; 5, a machining solution supplying unit for supplying a machining solution to the workpiece 1; 6, a numerical control unit for controlling the positions of the movable parts of the machine to perform a machining control operation and an automatic wire electrode returning operation; 7, an automatic wire electrode supplying unit for automatically connecting or cutting the wire electrode 4; and 8, a machining electric power unit for supplying electric power to cause electric discharge between the wire electrode 4 and the workpiece 1.
FIG. 2 is an explanatory diagram for a description of an automatic wire electrode returning operation which is carried out when the wire electrode is broken. In this case, the wire electrode is broken when it is on a path (e) after moving along paths (a), (b), (c) and (d). In FIG. 2, reference character (f) designates a wire electrode returning path which is extended from the point B where the wire electrode has been broken to the start point A.
FIG. 3 is a block diagram showing the internal control operation of the numerical control unit. In FIG. 3, reference numeral 9 designates an NC (numerical control) program for operating the machine body 3; 10, a program analyzing section for analyzing the NC program; 11, a machining solution controlling section for controlling the machining solution supplying unit 5 to supply the machining solution to the workpiece 1 and the wire electrode 4; 12, a wire electrode controlling section for turning on and off a wire electrode supplying operation, and for transmitting a wire electrode break signal and a contact signal representing the contact between the workpiece 1 and the wire electrode to a control section 13 (described later); 14, an automatic supply controlling section for controlling the automatic wire electrode supplying unit 7 to connect or cut the wire electrode 4; and 15, a machining input/output section for transmitting signals to and receiving signals from the machining solution controlling section 11, the wire electrode controlling section 12, and the automatic supply controlling section 14, to transmit the wire electrode break signal and the contact signal to the control section 13.
According to the output signal of the program analyzing section 10, the control section 13 applies a signal to a servo motor 16 to control the position of the movable table 2, and applies a signal to the machine input/output section 15 to perform the machining solution control, the wire electrode control, and the machining control. In addition, when the wire electrode is broken, the control section 13 performs all the control for automatically returning the wire electrode. Further in FIG. 3, reference numeral 17 designates a display controlling section for displaying data such as coordinates on a CRT (cathode ray tube) 18.
FIG. 4 is a flow chart for a description of the automatic wire electrode returning operation. In FIG. 4, reference numeral 19 designates a step of retuning the wire electrode to the initial hole A; 20, a step of causing the automatic wire electrode supplying unit 7 to connect the wire electrode 4; 21, a step of determining whether or not the wire electrode has been returned to the wire electrode break point B; 22, a step of setting a speed to return the wire electrode to the wire electrode break point B; 23, a step of returning the wire electrode along the locus to the wire electrode break point B 24, a step of starting the machining operation at the wire electrode break point B again; and 25, a step of continuing the machining operation.
The operation of the wire cut electric discharge machine thus constructed will be described with reference to FIGS. 1 through 4. The workpiece fixedly mounted on the movable table 2 is controlled in position by the numerical control unit 6, and is moved according to the NC program 9 analyzed by the program analyzing section 10. The machining wire electrode 4 and the machining solution supplying unit 5 are controlled by the machine input/output section 15. With the machining solution supplied by the machining solution supplying unit as an electric discharge medium, the electric discharge machining operation of the workpiece is carried out by using the machining electric power supplied by the machining electric power unit 8. The position of the movable table 2, and other data are displayed on the CRT 18 by the display controlling section 17.
Let us consider the case where the workpiece is machined as shown in FIG. 2. At the initial hole A, the wire electrode 4 is automatically connected by the automatic wire electrode supplying unit 7. When the wire electrode 4 is broken at the point B) on the path (e) after moving along the paths (a) through (d), the control section 13 applies an instruction signal to the automatic wire electrode supplying section so that the wire electrode 4 is cut at the point B, and then it is returned through the path (f) to the point A, where it is connected. Thereafter, the wire electrode is returned through the paths (a), (b), (c) and (d) to the point B, and the machining operation is carried out again.
The operation of returning the wire electrode from the point A to the point B is as indicated in the flow chart of FIG. 4. In step 19 the wire electrode is returned to the point A, and then in step 20 the wire electrode is connected at the point A. In step 21 it is determined whether or not the wire electrode has been returned to the point B. When it is determined that the wire electrode has not been returned to the point B yet, in step 22 a speed is set for returning the wire electrode to the point B, and in step 23 the movable table is moved along the locus. The speed set in step 22 has been determined by the control section in advance so that the wire electrode may not be broken again by being caught by the corner edges on the way to the point B. When the wire electrode is returned to the point B, in step 24 the machining operation is started again, and in step 25 the machining operation is continued.
The case where the wire electrode is broken for some reason will be described with reference to FIG. 5. When the wire electrode 4 is broken at the point B in FIG. 2, the sludge 100 formed during machining has been stuck to the machining region of the workpiece 1 including the upper and lower surfaces, and the portions of the first and second wire guides 30 and 3 which are confronted with the workpiece 1, as shown in FIG. 5. FIG. 5 is an enlarged diagram showing essential components in FIG. 1. The sludge 100 is formed when the workpiece 1 and the wire electrode 4 are molten by the machining thermal energy.
The conventional wire cut electric discharge machine is constructed as described above. Therefore, when the wire electrode is returned to the point A after broken, it is difficult to automatically connect the wire electrode because of the sludge 100 stuck to the machining region of the workpiece including the upper and lower surfaces and to the first and second wire guides4s 30 and 32. Even if the automatic wire electrode connecting operation has been achieved, while returning from the point A to the point B the wire electrode may be broken again by the sludge 100.
In the conventional method of returning a wire electrode in a wire cut electric discharge machine, as shown in FIG. 2 the wire electrode is returned from the wire electrode break point B to the initial hole A, where it is automatically connected, and then it is returned along the paths (a) through (e) to the wire electrode break point B, as was described above. Therefore, if the returning paths form acute angles, the wire electrode may be broken while moving along the paths. In order to overcome this difficulty, it is necessary to set the wire electrode returning speed to a relatively low value. This means that it takes a relatively long period of time to return the wire electrode to the break point; that is, the conventional method has a problem to increase the wire electrode returning speed.